Contactless identification systems or so-called radio-frequency-identification (RFID) systems typically include a base station or a reading device or a reading unit and a plurality of transponders or remote sensors. The transponders or their transmitting and receiving devices typically do not have an active transmitter for data transmission to the base station. Such inactive systems are called passive systems when they do not have their own power supply, and semipassive systems when they have their own power supply. Passive transponders draw the power necessary for their supply from the electromagnetic field emitted by the base station.
For data transmission between the transponder and the base station, for a programming operation of the transponder, for example, the transponder has an interface of a specific interface type, which is compatible with the corresponding interface type of the base station. The interface types can be divided, in a preliminary rough grouping, into contact and contactless types.
The interface types with which the data transmission occurs contactless or contact-free differ, inter alia, in the operating or carrier frequency used for the data transmission, i.e., the frequency transmitted by the base station. Frequently used frequencies are, for instance, 125 kHz (LF range), 13.56 MHz (RF range), a frequency range between 860 MHz to 960 MHz (UHF range), and a frequency range greater than 3 GHz (microwave range).
Another differentiating feature of the different interface types is the type of coupling between the specific interfaces of the transponder and the base station. In this case, inter alia, the so-called inductive or magnetic coupling and the so-called far-field coupling are differentiated. Described in simplified terms, in inductive or near-field coupling, an antenna coil of the base station and an antenna coil connected to the input circuit of the transponder form a transformer, which is why this type of coupling is also called a transformer coupling. In inductive coupling, a maximum distance between the transponder and the base station is limited to the near field of the employed antenna. The near-field range is substantially established by the operating frequency of the interface.
The so-called load modulation is usually used in inductive coupling for data transmission from a transponder to a base station; in this regard, see, for example, Finkenzeller, Chapter 3.2.1.2.1 “Load Modulation.”
For data transmission from the base station to the transponder, in inductive coupling, the base station usually transmits a carrier signal with a frequency in a frequency range of 50 kHz to 250 kHz. To begin the data transmission, the base station via amplitude modulation of the carrier signal first generates a short field gap or a so-called “gap”; i.e., the amplitude of the carrier signal is damped or attenuated briefly, for example, for about 50 μs to 400 μs, or totally suppressed.
Characters which are transmitted subsequent to the initiation of the data transmission by the base station are encoded by the associated durations between temporally successive field gaps. A first character value is hereby assigned a first duration and at least one second character value is assigned a second duration. To decode the transmitted characters, the transponder determines the specific durations between the field gaps and determines the value of the transmitted character from the determined duration.
For error-free data transmission or decoding of the characters, it is necessary that the signal courses generated by the base station and received by the transponder by inductive coupling have established maximum tolerances, for example, in regard to their time course and/or employed level.
To increase the achievable ranges between base station and passive transponders, the quality of a parallel resonant circuit, which is formed here by the antenna coil and a capacitor connected parallel thereto, is increased in order to enable the supplying of the passive transponder from the field transmitted by the base station at greater distances as well. The reduced damping of the resonant circuit has the effect that at a field gap the coil voltage or the voltage of the parallel resonant circuit of the transponder declines more slowly than in the case of a resonant circuit with a lower quality and therefore higher damping. Because the field gap in the transponder can be detected, however, only when the coil voltage or a voltage obtained from the coil voltage by rectification has declined below a settable potential, field gaps can be detected in a delayed manner in comparison with a resonant circuit with a lower quality. This has the result that the duration of the field gaps detected in the transponder are shortened and the durations between the field gaps, which represent the corresponding character value, are lengthened. This change in the timing of the signal courses detected in the transponder is influenced directly by the quality of the resonant circuit. In other words, the timing of the signals received in the transponder substantially depend on various parameters, for example, on the employed antenna coil, as a result of which an error-free data transmission cannot always be guaranteed in the case of changes in parameters.
In a conventional system, to be able to ensure interference-free data transmission also in the case of such parameter-dependent timing variations, after the initiation of the data transmission, a reference duration is transmitted by successive field gaps by the base station, with which a calibration value is determined in the transponder, whereby the calibration value is used for calibrating the subsequently received durations. The reference duration in this case corresponds to a known character value, for instance, “0.” Because it is known in the transponder how long the duration belonging to the character value “0” must be theoretically, the calibration or offset value can be calculated from the actual, measured reference duration.
However, this method cannot be carried out with transponders not supporting this method or this transmission protocol, because these interpret the reference duration already as a character, as a result of which the character sequence received in the transponder is corrupted.
German Patent Publication DE 198 27 476 C1 discloses a method in which after an HF charge pulse two reference pulses are transmitted, of which the one with a longer duration represents an H-bit and the other with the shorter duration an L-bit.
German Patent Application DE 197 44 781 C1, which corresponds to U.S. Pat. No. 6,044,333, discloses a method for calibrating an RC oscillator of a transponder, in which a data set is expanded by a calibration signal to calculate a correction value.